Recently, automobile manufacturers have increasingly used lightweight, high-strength materials as materials for automobiles to prevent environmental pollution and improve the fuel efficiency and safety of automobiles, and such lightweight and high-strength materials have also been used as materials for automotive structural members.
In the related art, high-strength steel sheets formed of low carbon steel having a ferrite matrix have been used as steel sheets for automobiles. Although low-carbon, high-strength steel sheets are used to manufacture automobiles, it has been difficult to obtain commercially-viable low-carbon, high-strength steel sheets having a maximum elongation of 30% or greater if the low-carbon, high-strength steel sheets have a tensile strength of about 800 MPa or greater. Therefore, it is difficult to use high-strength steel sheets having a strength of about 800 MPa or greater for manufacturing complex components. That is, the use of such high-strength steel sheets only allows for the manufacturing of simple components and makes it difficult to manufacture freely designed components.
In addition, when current steel sheet manufacturing techniques are considered, it is difficult to manufacture steel sheets having a high degree of strength on the level of 1300 Mpa or greater and processable through a cold pressing process or a roll forming process.
Patent Documents 1 and 2 have proposed methods for solving the above-mentioned problems. Patent Documents 1 and 2 disclose high-manganese austenitic steels having high degrees of ductility and strength.
In Patent Document 1, a large amount of manganese (Mn) is added to steel to obtain a steel sheet having a high degree of ductility. However, work hardening occurs severely in deformed portions of the steel sheet, and thus the steel sheet is easily fractured after being worked. In addition, although Patent Document 2 provides a steel sheet having an intended degree of ductility, the characteristics of the steel sheet for electroplating and hot dip plating are poor because of the addition of a large amount of silicon (Si). Furthermore, although Patent Documents 1 and 2 provide steel sheets having high degrees of workability, the yield strength of the steel sheets is low, and thus the crashworthiness of the steel sheets is poor. Moreover, since the steel sheet disclosed in Patent Document 2 has poor weldability in three-sheet lap welding, poor delayed fracture resistance, and a degree of strength on the level of 1200 MPa or less, the marketability of the steel sheet was low, and the steel sheet was not successfully commercialized.
In addition, automobile manufactures have recently increased the use of twining-induced plasticity (TWIP) steel because the formation of twins in high-manganese steel during plastic deformation increases the work hardening of high-manganese steel and thus the formability of high-manganese steel.
However, there is a limit to increasing the tensile strength of TWIP steel containing austenite, and thus it is difficult to manufacture a ultrahigh-strength steel sheet using TWIP steel.
(Patent Document 1) Japanese Patent Application Laid-open Publication No.: 1992-259325
(Patent Document 2) International Patent Publication No.: WO02/101109